The Arlington School Board decided last night to consider adding a seventh period to high school students' schedules, a move proposed this week by Superintendent Arthur W. Gosling.

Although the board did not vote to implement the change, it gave the superintendent the go-ahead to include funding for the extra period in next year's budget request to the County Board, which finances the schools.

Only Montgomery County and the City of Falls Church require students to take seven classes a day.

"This is an evolutionary thing," said Judy Connelly, board vice chairman.

Last night Gosling told the board that it would cost $438,000 to $563,000 to add another period. He also outlined a plan under which only 10 minutes would be added to the school day to accommodate the extra period. Gosling proposed to reduce each class period from 51 minutes to 47 and shorten the lunch period from 51 minutes to 36.

He said the change would require an additional 14 to 18 teachers. School officials have said that 28 high school teachers would lose their jobs because of declining enrollment next year. The extra period would reduce the number of layoffs.

Gosling said the growing number of courses required by the state and more rigorous college admissions requirements are making it impossible for students to take traditional elective courses, such as art and music.

The change dovetails with a new science curriculum under consideration by the board. The program would enhance science lessons in secondary schools and create a host of advanced courses, seminars and training on sophisticated equipment for talented students who are accepted into the new elective program.

At the same time, Gosling's plan emphasizes flexibility in the kind of courses students will be able to take, including study hall and vocational, versus academic, courses.